The answer: When it's a convertible system that can be twisted or folded to create several different computing personas, each aimed at enabling a different kind of computer interaction.

Rather than being a single-purpose laptop where the keyboard and screen are front and center, convertibles can morph into as many as five different computing configurations, each with a distinct personality. These "personalities" can include a traditional laptop, a tablet, a system that folds flat on a table, one with a tent-like inverted "V" profile and a presentation machine with the screen facing away from the keyboard.

Using a convertible can be a liberating experience. Instead of locking you into a particular way of using the computer, a convertible offers the flexibility to let the system fit the task at hand. Convertibles have specially-designed hinges that allow the systems to change into different configurations as if they were Transformers.

For example, after you've finished creating that career-boosting presentation using a traditional laptop orientation, flip the screen over to show it to your coworkers. Later, fold the system flat on your desk to sketch a map while having the keyboard handy to type directions. When the work is done, fold the keyboard out of the way so you have a tablet for Web work, ebook reading or just playing games.

To see just how flexible this genre of mobility really is, I've tried out three of the newest convertibles on the market: the Dell XPS 11, the HP EliteBook Revolve 810 G2 and the Sony Vaio Fit 11A | Flip PC. I've also included videos showing exactly how each configuration works.

Different systems, different configurations

Each of these machines approaches convertibility in a different way.

The HP EliteBook Revolve has a central hinge that rotates and swivels to allow the screen lid to -- as the name implies -- revolve 180 degrees and then fold flat. This allows it to convert from a laptop into a presentation machine, fold flat on a desk or work as a handheld tablet.

The Dell XPS 11 has a pair of hinges that can rotate the display around 360 degrees. This way, the device can assume the guise of a laptop or a presentation machine, fold flat or become a tablet.

The Sony Flip PC has a long hinge that lets the screen open to 140 degrees, while a second hinge in the middle of the display's frame lets you pivot the screen forward or backward so that the system can transform from a laptop into a presentation system or a tablet.

Otherwise, these three convertibles have a lot in common. They all include an 11.6-in. touchscreen, SSD storage and Windows 8.1 software. At between 2.5 lb. and 3.1 lb., they are reasonably average size for laptops. They are, however, far heavier than most standalone tablets, such as Samsung's 12.2-in. Galaxy Note Pro, which weighs 1.7 lb. As a result, some convertibles can be tedious to hold for long periods.

None of them is low-cost -- the extra design, engineering and manufacturing for the specialty hinges make these systems expensive compared to conventional laptops, with costs running between $800 and $1,829.

Still, if you like the idea of your laptop being a Swiss army knife that can change into a tablet, fold flat on a table or have its screen point away from the keyboard, a convertible is the best game in town.

Dell's XPS 11 is a thin, lightweight system that has a beautiful ultra-high-resolution screen. However, some of its other features -- such as the keyboard -- don't quite live up to its promise.

The XPS 11 has four distinct computing personalities. In addition to being a laptop, it folds flat on a tabletop, can be opened like a tent or used as a tablet.

The simplest of the three convertibles here, the XPS 11 has a pair of hinges that can flip the display a full 360 degrees, so that it easily changes from a laptop to a tablet. The hinge action is smooth and the magnetic clasps are strong enough to hold the system closed in both its laptop and tablet configurations.

Dell XPS 11

Made of stainless steel, the XPS 11's hinges have, according to Dell, survived testing that includes more than 20,000 rotation cycles. Based on an average of 10 rotations per day, that should translate into an estimated lifetime of at least five and a half years.

The XPS 11 is 0.5 in.-thick when used in laptop mode and 0.4 in.-thick in tablet mode. With a footprint of 11.8 x 7.9 in and a weight of 2.5 lb., the XPS 11 is both the smallest and the lightest of the three convertibles reviewed here, and was the easiest to hold and use for extended periods as a tablet. The case is constructed of a lightweight carbon fiber base with a comfortable, soft finish and a magnesium palm rest area.

The system's WQHD 2560 x 1440 display surpasses the displays used on the HP EliteBook Revolve G2 and the Sony Flip 11 in its ability to render detail. However, I didn't find it as bright as the Revolve G2's lower-resolution display.

Made of Gorilla Glass NBT, the display should be able to stand up to daily use; it responds well to 10 individual finger inputs. However, when used as a laptop, the screen noticeably wobbles. I found that I needed to brace the screen with my left hand while tapping or swiping with my right.

The XPS 11's biggest compromise, though, is its keyboard. Rather than the traditional mechanical setup that's used on the HP EliteBook Revolve and the Sony Flip 11, the XPS 11 has membrane-based keys that provide little or no tactile feedback. For a touch typist, it's better than using a screen-based keyboard, but not by much. The keys' sensitivity is adjustable, but to type accurately, I needed to slow down from about 40 to 45 words per minute to about 15 or 20 words per minute in order to deliberately strike the keys.

When it's set up as a tablet, the keyboard and touchpad are facing downward, which made holding the tablet (with the uneven surface of the keyboard against my hand) feel a bit weird. To prevent inadvertent typing, the keyboard and touchpad automatically turn off in tablet mode.

The XPS 11 is outfitted with a pair of USB 3.0 ports, an HDMI port and an audio jack. It offers 802.11n Wi-Fi and Bluetooth. The system has a Trusted Platform Module for business security, while there's a thoughtfully-placed NFC connection spot built under the touchpad. I found it much easier to use than the NFC spots on the other two that are located in what is the bottom of the system in laptop mode.

The base system costs $1,000 and comes with an Intel Core i3 processor, 4GB of RAM and an 80GB SSD. There are two other configurations: one with an Intel Core i5 processor and a 128GB SSD for $1,250 (the model sent for this review) and an Intel Core i5 model with a 256GB SSD for $1,450.

Test results

The XPS 11's performance was middle of the pack with a respectable PassMark PerformanceTest score of 1,311.3 -- 40% lower than the Revolve G2's score. It scored a 157 on CineBench's processor test, but it couldn't complete the graphics portion (as was the case with the Revolve G2; this was due to a driver problem that Intel is working on).

Lasting for four hours and 35 minutes of continuous HD video playback, the XPS 11's battery life should translate into a full day of on-and-off use with some power conservation. This was nearly three hours less than the Revolve G2's battery life but an hour longer than the Sony Flip's battery life. The battery isn't interchangeable.

The XPS 11 came with Windows 8.1 along with Tagaboom, a program that lets you turn photos, music and voice-over narration into multimedia shows. It comes with a one-year warranty; extending it to three years costs $200.

Bottom line

Designed and built primarily as a tablet, the XPS 11 fulfills that role well. But because its keyboard falls short, the XPS 11 doesn't work as well as a laptop.

Rather than a dual-hinge design, the EliteBook has a single stainless-steel hinge (based on designs used on earlier HP convertibles such as the EliteBook 2740p ) that smoothly rotates and swivels to deliver four distinct orientations. In addition to a conventional laptop, the EliteBook can be a tablet or a presentation system, or can lie flat on a desk.

If you grab the top of the screen and rotate it clockwise, the Revolve G2 is transformed into a presentation system with the screen facing away. You can then fold the display flat over the keyboard with the screen facing up to make it a tablet. The whole unit can also be folded flat on a tabletop.

HP EliteBook Revolve 810 G2

When it is folded flat, the Revolve G2 is functional, but because the hinge is raised off the surface of the table, the display slopes down and away from the user at a 5-degree angle, making it awkward to use.

This turned out to be the rare system that doesn't noticeably wobble when the display is tapped in laptop mode -- as opposed to the other two systems reviewed here, which often needed to have the screen braced while using them.

However, my feelings about the Revolve G2 as a tablet are mixed. While it has a handy 0.7-inch lip on the side that makes it easier to grab and hold without touching the screen, at 3.1 lb. (half a pound more than the Dell XPS 11), it is too heavy for prolonged use. It also lacks the Sony Vaio Flip's 5-degree tilt for desktop work. And the magnetic clasps that hold the screen lid in place when it's closed or in tablet mode are too strong -- it actually takes considerable effort to pull them apart.

According to HP, the hinge survived 50,000 open-close cycles and 25,000 full rotations in tests. If the Revolve G2 goes through an average of 10 rotations a day, this translates into an estimated lifetime of at least 6.8 years for the hinge.

Made of magnesium, the case has a soft coating on the bottom that feels good and makes it harder for the device to slip out of your hands when it's in tablet mode. The screen lid and keyboard area have a roughened feel to them, though, and the contrast between the surfaces can be disconcerting.

The Revolve G2's 11.6-in. screen is made of Gorilla Glass 3, but its 1366 x 768 resolution pales in comparison to the XPS 11's 2560 x 1440 display. On the other hand, I thought it was the brightest of the three.

The Revolve G2 can respond to 10 individual finger inputs, but you can also use HP's $49 Executive Pen Gen 2 for more precise work. The 1-oz. stylus responds to 256 levels of pressure and uses an easy-to-find AAA battery.

In its laptop mode, the Revolve G2 measures 1.0 x 11.2 x 8.3, making it the largest of the systems reviewed here. In addition to a pair of USB 3.0 ports and an audio port, the Revolve G2 has a DisplayPort (which will need a converter if you want to use it as an HDMI port). It has 802.11n Wi-Fi and Bluetooth as well as an Ethernet port (the only one of the three reviewed here that includes one). The system has a Trusted Platform Module as well as an NFC connection spot on the bottom of the system.

The Revolve G2 comes in three models. The basic permutation offers an Intel Core i3 processor with 4GB of RAM and a 128GB SSD for $1,299. The next model upgrades to an Intel Core i5 processor for $1,599, while the top of the line (which was the model I reviewed) included an Intel Core i7 processor, 8GB of RAM and a 256GB SSD for $1,829.

Without a doubt, $1,829 is a lot to spend on a laptop, but the Revolve G2 comes with a three-year warranty, while the others reviewed here are covered for one year; it includes HP's Elite Care, which routes service requests to specially-trained agents.

Test results

Built for all-out performance, the Revolve G2 outpaced the other two convertibles on the PassMark PerformanceTest suite. It scored 258 on CineBench's processor tests, but like the XPS 11, it couldn't complete the graphics tests due to a driver incompatibility that Intel is working on.

With all that power available, it is amazing that the system's 3,800mAh battery pack ran for seven hours and 20 minutes of constant use on a charge. This is nearly three hours longer than the XPS 11's results and twice the run time of the Vaio Flip. With modest power management settings in place, this means the Revolve G2 could probably last for over 13 hours of normal usage. It is also the only laptop of the three where you can swap the battery for a fresh one.

In addition to Windows 8.1 Pro, the Revolve G2 comes with some cool HP utilities. My favorite is PageLift, which straightens a skewed photo of a document taken with the system's webcam to something that's presentable. It also includes the PDF Complete Acrobat file viewer and a slew of security utilities.

Bottom line

While the flexibility of the Revolve G2's four computing personas would easily fit in at work or at home, its size and weight make it hard to comfortably use as a tablet. However, if you don't mind the extra heft, this is a very versatile mobile computer.

The newest of the convertibles reviewed here, Sony's Vaio Fit 11A | Flip PC (and yes, that is the complete name) is built to be equal parts laptop and tablet. The Fit 11a has the most complicated convertible hardware of the three and delivers a trio of distinct computing personas.

In addition to the expected laptop configuration, the Fit 11a can be turned into a presentation system with the screen facing away or a wedge-shaped tablet. Unlike the others, it can neither fold flat on a table nor work in a tent-like configuration.

The system has a long hinge that allows the display to open up to 140 degrees in standard laptop orientation. There's also a second pair of hinges midway up the frame that supports the display, allowing the display to flip around the top of that frame so that it can also face away from the keyboard. According to Sony, the Fit 11a's hinge design is patent-pending. The company wouldn't disclose any details as to its construction or testing, except to say that it has been tested for durability.

Sony Vaio Fit 11A | Flip PC

The system has a screen lock that prevents the screen from inadvertently flipping over while you're using it.

I found this pair of hinges to be a lot to deal with -- I had to make sure the screen was nearly vertical when flipping the screen or it banged into the base. At times, I wished I had an extra hand to better convert the system from one configuration to another.

As is the case with the Revolve G2, the Fit 11a has a magnetic clasp, but it has just enough holding power to keep the screen lid in place in tablet or closed-laptop mode. As a result, it is easier to work with than the Revolve G2.

On the other hand, the display wobbled a lot when I tapped or swiped along its surface. As a result, I frequently found that I needed to use my left hand to steady the display while I was tapping with the right one.

In a rare design faux pas for Sony, the Fit 11a's volume control (which is on the far right of the back edge) is covered by the screen when it is set up as a laptop or presentation system. As an alternative, you can adjust the volume using the system's function keys.

The system has an aluminum screen lid and a plastic base, but it lacks the XPS 11's soft coating. At 2.8 lb., it is a little heavy to hold with one hand for extended periods of time as a tablet.

In tablet mode, the system is wedge-shaped, and it has a 0.5-inch lip to grab onto without accidentally touching the screen. Still, the Fit 11a felt awkward in the hand, particularly when held vertically. On the other hand, this gives the tablet a nice 5-degree tilt when it's on a tabletop, making it very finger friendly.

The 11.6-in. screen can show 1920 x 1080 resolution, second best to the XPS's ultra-high-definition display; I found it midway between the other two convertibles in terms of brightness. It is not made of Gorilla Glass, as are the XPS 11 and Revolve G2, but the screen is scratch resistant.

As with the others, the Fit 11a's screen can interpret up to 10 independent touch inputs. Sony includes a slim stylus made by N-trig that weighs 0.6 oz. It is precise and can interpret 256 levels of pressure (although it uses a relatively hard to-find AAAA battery). I liked the stylus -- it was comfortable, quite accurate and much better than the HP stylus or an off-the-shelf one.

The Fit 11a comes with a pair of USB 3.0 ports, an HDMI port, an audio connection, 802.11n Wi-Fi, Bluetooth and an NFC connection spot on the bottom. It doesn't come with an Ethernet port or a Trusted Platform Module.

At $800, the Fit 11a is a bargain. Unlike the others, there are no other configurations for this display size: it is offered only with the Pentium quad-core N3520 processor, 4GB of RAM and a 128GB SSD. Sony does sell larger Flip laptops with 13.3-in., 14-in. and 15.5-in. screens.

Test results

The Fit 11a's score of 909.0 on the PerformanceTest benchmark was less than half that of the Revolve G2, but I was still impressed that it did so well, considering its less powerful hardware. For writing, Web excursions and even some gaming, the Fit 11a should be more than enough, but may disappoint on more processor-intensive tasks, like video editing.

On CineBench's processor tests, the Fit 11a scored an impressive 146 points, only 7% behind the XPS 11, which has a more powerful processor. Unlike the Revolve G2 and the XPS 11, the Fit 11a uses Intel's HD Graphics accelerator and as a result was able to complete the CineBench graphics tests; it had a midrange result of 2.3 frames per second.

Its battery life of three hours and 32 minutes, timed after continuously playing videos, was acceptable but unimpressive compared to the Revolve G2. I estimate that this means the Fit 11a should last for about seven hours of normal use with some power management. Unlike the Revolve G2, you can't change the battery.

In addition to Windows 8.1, the Fit 11a includes full copies of Adobe Photoshop Elements 12 and ArtRage Studio. The system comes with a one-year warranty; if you add $180 for a three-year warranty upgrade, the Fit 11a is still an economical $980.

Bottom line

A reasonable compromise between the competing interests of a laptop and a slate, the Fit 11a is a bargain, but its double-hinge mechanism is needlessly complex and doesn't allow it to convert into enough computing personas to make it a winner.

Each of these laptops is able to convert from a laptop to a presentation machine to a tablet, although all come at it from a different angle.

The HP EliteBook Revolve 810 G2 is a portable powerhouse that mixes excellent battery life with impressive performance. I found that it worked better as a laptop or presentation system than a tablet or fold-flat system; in addition, at more than a half-pound overweight, it's tedious to hold as a tablet. And even the entry-level models are surprisingly expensive compared to the competition.

Frankly, I was wowed by the Dell XPS 11's ultra-high-resolution screen, its sleek profile and low weight. Easily the best of the bunch at being a tablet, the XPS 11's soft skin is inviting to the touch.

I did miss having the stylus option. But more to the point, the XPS 11's lackluster keyboard provides a substandard typing experience and is a step back in terms of the evolution of mobile computing.

Finally, despite having the most complicated hinge design of the three convertibles, the Sony Vaio Fit 11A | Flip PC offered the least number of movement configurations of the three laptops. As a tablet, I found it awkward to hold vertically, while as a laptop, the screen wobbled noticeably and the volume control is hidden behind the display. The Fit 11a does, however, have a nice 5-degree tilt when it's set up on a desk as a tablet.

So all in all, while each of these convertibles is an impressive feat of engineering and design, none of them went obviously to the top of my list. Unless you're really eager to purchase, I'd wait until the next generation of convertibles comes out. They will hopefully be thinner, lighter, better designed and more versatile.

How I tested: Convertible laptops

To see how these convertible laptops compare with one another, I used them in a variety of locations for several different typical tasks over three weeks. Each accompanied me on a short trip.

I spent some time getting to know each system, examining every major aspect. I tried them out in their different configurations, paying attention to how hard it is to convert from one computing personality to another, how well they compare to single-purpose laptops and tablets as well as if any of the controls are inaccessible in any of the configurations.

I tried each out in laptop mode, and I determined how much wobble the screen has when it is swiped or tapped. Then, I turned it into a tablet and carried it around, worked on a tabletop and on my lap. This was followed by use as a presentation machine, folding the screen flat on a table and finally in tent mode, if the system supported it.

I used each as a touchscreen and with a Wacom Bamboo stylus as well as whatever stylus was included with the system. To gauge if it could work with 10 individual inputs, I opened Paint and drew all 10 of my fingers across the screen. Then, I drew a map and sketched some geometric figures on the screen.

I measured the thickness of each system with a digital caliper and then measured the length and width in its laptop and slate configurations. I weighed each on a digital scale with and without its AC adapter. I connected each to a home and a public Wi-Fi network. I tested whether the system had a Near Field Connection chip (NFC) by using a Nexus 7 with the NFC Tools app.

I tested overall performance with the PassMark PerformanceTest 8.0 benchmark suite of tests. I ran the software three times and averaged the results. I also ran the Maxon CineBench benchmarks for graphics and processor performance; I averaged the results of three runs.

To gauge how long each can run on its battery, I loaded the PassMark BatteryMon test, fully charged the system, set its power management options to Balanced and adjusted the settings to prevent the computer from going to sleep. I used the shuffle feature on Windows Media Player to continuously play six videos from a USB flash drive connected to the system and reported the average of three runs.

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